Communication systems that utilize coded communication signals are well known in the art. One such system is a code division multiple access (CDMA) cellular communication system such as set forth in the Telecommunications Industry Association/Electronic Industries Association International Standard (TIA/EIA IS-95), hereinafter referred to as IS-95. In accordance with the IS-95, the coded communication signals used in CDMA systems comprise CDMA signals that are transmitted in a common 1.25 MHz bandwidth to base stations of the system from mobile or wireless communication units, such as cell phones or portable wireless computers or wireless handheld devices, that are communicating in a specific coverage area of the base station. In conventional CDMA systems, a base station transceiver subsystem (BTS) communicates with a base station controller (BSC) which allows the communication unit to communicate with other communication units within the same coverage area. Each CDMA signal includes a pseudo-noise (PN) sequence associated with a particular base station and an identification number of a communicating communication unit.
Typically, the BSC is connected to a mobile switching controller (MSC) which allows a BTS to connect with other BTS outside its coverage area in order to allow a communicating communication unit communicate with other units outside its coverage area.
FIG. 1 illustrates a conventional CDMA communication system 100 including a first base station 110, a second base station 120, and one or more communication units 105, 106. The communication system 100 illustrated in FIG. 1 is an exemplary CDMA system which includes a direct sequence CDMA cellular communication system, such as that set forth in TIA/EIA IS-95.
In the system shown in FIG. 1, base stations 110 and 120 are connected to base station controller 130 and mobile switching controller 140 which is in turn are connected to public switched telephone network (PSTN) 150 using known techniques.
The system shown in FIG. 1 further connects to the public land mobile network (PLMN) and PSTN to allow mobile communication units to travel from one network (roaming) to another network while maintaining a subscriber profile information. A detailed illustration of the PLMN is shown in FIG. 1B. In the system shown in FIG. 1B, a conventional cellular (or PCS) wireless communication network is shown. In the network shown in FIG. 1B, a network subscriber's profile information is typically stored and maintained in a home location register (HLR) and a visitor location register (VLR).
Still referring to FIG. 1, when a communication unit initiates a call sequence to either one of base stations 110 and 120 within a coverage area, an end-to-end connection is established between the respective base station and base station controller 130 and MSC 140 using known CDMA call setup techniques. Base stations 110 and 120 typically communicate with BSC 130 and MSC 140 via communication links, such as a T1 connection. Base stations 110 and 120 typically have antennas to define the coverage area within which either base stations primary accommodate the communication units.
In the system shown in FIG. 1, when a communicating communication unit initiates a call sequence (uplink) to the nearest base station, the call is assigned to the target communication unit via BSC 130 and MSC 140 within a prescribed bandwidth (e.g. 1.25 MHz for IS-95).
Also, in the conventional CDMA system shown in FIG. 1, communication between a communicating communication unit and the base station requires a dedicated end-to-end connection between the base station, the BSC and the MSC. Such dedicated end-to-end connection can also be very expensive and time-consuming.
In the exemplary CDMA system shown in FIG. 1, each base station transmits a pilot signal having a common PN spreading code that is offset in code phase from the pilot signal of other base stations within the system. During system operation, the mobile communication unit is provided a list of code phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The mobile unit is equipped with a searching function which allows the mobile unit to track the signal strength of the pilot signal from a group of base stations including the neighboring base stations.
Various methods exist for switching the mobile communication unit from one base station to another (typically known as “handoff”). One such method is termed a “soft” handoff, in which communication between the mobile unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This method is considered a soft handoff in that communication with the subsequent base station is established before terminating communication with the original base station. When the mobile unit is communicating with two base stations, a single signal for the end user is created from the signals from each base station by a communication system controller.
Mobile unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the communication unit. An active set is the set of base stations through which active communication is established. A neighbor set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication.
When communications are initially established, the communication unit communicates through a first base station, and the unit monitors the pilot signal strength of the base station in the active set and the neighbor set. When a pilot signal of a base station in the neighbor set exceeds a predetermined threshold level, the base station is added to the candidate set and removed from the neighbor set at the communication unit.
The communication unit communicates a message identifying the new base station. The BSC decides whether to establish communication between a new base station and the communication unit. Should the BSC decide to do so, the BSC sends a message to the new base station with identifying information about the communication unit and a command to establish communications.
When the communication unit is communicating with multiple base stations, it continues to monitor the signal strength of base stations to determine which base station to connect to in the event of a signal strength degradation.
Each base station has a coverage area that has two handoff boundaries. A handoff boundary is defined as the physical location between two base stations where the link would perform the same regardless of whether the mobile unit were communicating with the first base station or the second base station. Each base station has a forward link handoff boundary and a reverse link handoff boundary.
The forward link handoff boundary is defined as the location where the mobile unit's receiver would perform the same regardless of which base station it was receiving. The reverse link handoff boundary is defined as the location of the mobile unit where two base station receivers would perform the same with respect to that mobile unit. Ideally these boundaries should be balanced, meaning that they have the same physical location, with respect to the base station. If they are not balanced, system capacity may be reduced as the power control process is disturbed or the handoff region unreasonably expands.
In any of these conventional systems, the soft handoff between base stations still require the active base station to maintain contact with the BSC as it hands off communication to a neighboring base station or a candidate base station. Upon handing over communication, the new base station (now active base station) resumes communication with the mobile unit via the BSC. The conventional system described in FIG. 1 or FIG. 2 does not allow each base station to communicate with the other during a handoff since all communication has to go through the BSC. This takes time and in a data traffic transmission can be costly.
With the introduction of enterprise in building wireless communication services, the problem of handoff is even more accentuated within the enterprise system. FIG. 3 is a prior art illustration of an enterprise communication system. In the system of FIG. 3, the enterprise system may comprise of multiple BTS subsystems which may be located at the lower floor of a multi-floor building or campus networks.
The enterprise system of FIG. 3 needs to co-exist with the wide-area public system in terms of frequency reuse, interference control and to provide seamless services via handoff. However, in the system of FIG. 3, a common problem occurs when a mobile user enters a location that is near a border between two cell sites within the enterprise system. In this situation the signal level of the mobile unit tends to fluctuate at both cell sites. This signal level fluctuation results in a ping-pong situation in which repeated requests are made to handle calls back and forth between the two cell sites. Furthermore, the ping-pong situation raises the possibility that the call will be discontinued if it is unnecessarily transferred to a cell in which all channels are in use and thus unavailable for accepting the handoff.
In the system illustrated in FIG. 3, a user located in a higher floor in a multi-floor building may experience unnecessary ping-pong handoffs as a result of the mobile unit trying to communicate with the BTS internal to the enterprise system and the external cell sites which may be transmitting stronger signals than the internal BTS serving the mobile unit. This is because the signal strength of the wide-area public system is stronger, especially at locations “Loc A” and “Loc B” than the signal strength from the BTS located on the ground floor of the building.
Such unnecessary ping-pong of handoffs increases the mobile unit incorrectly hearing handoff commands or failure in hearing commands. Furthermore, the ping-pong situation raises the possibility that a call will be discontinued. This can be very costly and time-consuming for a mobile user communicating with any other mobile user within the enterprise system. Unnecessary ping-ponging handoffs also make it costly for the enterprise system accounting to reconcile calls within the enterprise system and those made to external cell sites.
Therefore, it is desirable to have a robust method for handing-off CDMA calls including voice and data over a communication pathway within an enterprise wireless communication system. It is further desirable to have a CDMA call handling method that handles the transmission of calls, especially data calls, without the inherent costly call ping-pong effect of the prior art. A need further exists for an improved and less costly system which improves the efficiency and the transmission rate and time of calls between a mobile unit and a base station and between base stations and a base station controller and between adjacent base stations within an enterprise wireless communication system.